A central question in the field of attention is whether visual processing is a strictly limited resource, which must be allocated by selective attention. If this were the case, attentional enhancement of one stimulus should invariably lead to suppression of unattended distracter stimuli. Here we examine voluntary cued shifts of featureselective attention to either one of two superimposed red or blue random dot kinematograms (RDKs) to test whether such a reciprocal relationship between enhancement of an attended and suppression of an unattended stimulus can be observed. The steady-state visual evoked potential (SSVEP), an oscillatory brain response elicited by the flickering RDKs, was measured in human EEG. Supporting limited resources, we observed both an enhancement of the attended and a suppression of the unattended RDK, but this observed reciprocity did not occur concurrently: enhancement of the attended RDK started at 220 ms after cue onset and preceded suppression of the unattended RDK by about 130 ms. Furthermore, we found that behavior was significantly correlated with the SSVEP time course of a measure of selectivity (attended minus unattended) but not with a measure of total activity (attended plus unattended). The significant deviations from a temporally synchronized reciprocity between enhancement and suppression suggest that the enhancement of the attended stimulus may cause the suppression of the unattended stimulus in the present experiment.human EEG | neural mechanisms of shifting | steady-state visual evoked potentials | feature-based attention | random dot kinematogram S hifting and focusing attention on a certain location, object, or feature is a key element in the extraction of sensory information to allow for adaptive behavior. The distribution of attentional resources and the underlying temporal neural mechanisms of attentional shifting are still not well understood. Some previous studies have measured event-related potentials (ERPs) during the cue-target interval (1-5). This approach, however, allows one to investigate only the neural mechanisms of cue processing and target expectation and, thus, the activity of a cortical control network (1); it cannot provide information on the temporal dynamics of neural facilitation and/or suppression in early visual processing areas that are involved in the processing of the new to-be-attended visual stimulus. Knowledge of these dynamics in early visual processing areas seems pivotal for understanding the effect of top-down control mechanisms in attentional shifts because behavioral performance is closely linked to the modulation of the cortical evoked activity representing the newly attended stimulus (6). If visual processing were a strictly limited resource, which is distributed by selective attention, enhancement of an attended stimulus should be accompanied by an equal suppression of the unattended stimulus. However, in previous studies, we showed that shifting attention to either one of two lateral flickering stimuli was purely facilitatory, with ...